I believe this is because the code `counts[word]++` basically works by:

    1. Hash word, retrieve the value from the counts map
    2. Increment the value
    3. Hash word, store the value into the counts map

By change counts to a map of strings to pointers of ints, you only need to perform #3 once per unique word. Once you've read the int pointer, you need only write to the pointer and not re-hash the word and write that to the map, which can be expensive.

https://github.com/benhoyt/countwords/blob/master/optimized....